When crude oil is extracted from a reservoir, it contains water and salts. At the high temperatures that may be encountered in a refinery during crude oil processing, the water can hydrolyse the salts to form corrosive acids. Chloride salts are typically found in crude oil and pose a particular problem, since they can form hydrochloric acid. Bromide salts can also be found, and they can form hydrobromic acid.
Over time, corrosive acids can cause significant damage to refinery equipment. Damage is commonly observed in the lines that transport crude oil from one area of a refinery to another. Considerable time and cost may be involved in replacing damaged refinery equipment. In some cases, for instance where a bypass pipe has not been provided, processing of the crude oil will need to be stopped entirely in order for the refinery equipment to be replaced.
It is therefore desirable for salts to be removed from hydrocarbon fluids such as crude oil before refinery processing. To solve this problem, crude oils are passed to a desalter before they are processed in a refinery.
Crude oils are typically mixed with wash water before they are passed to the desalter. Once introduced into the desalter, a desalted crude oil phase and an aqueous phase form. The aqueous phase contains water (that which was present in the extracted crude oil, as well as water that has been added to the hydrocarbon stream during processing, such as wash water) and salt. A rag layer separates the two phases. The rag layer is a mixture of the aqueous phase and the desalted crude oil phase.
A desalted crude oil stream and an aqueous stream are withdrawn from the desalter through separate lines. The streams are typically withdrawn at points in the desalter which are a distance from the rag layer so as to minimise the presence of any aqueous components in the desalted crude oil stream and vice versa.
Methods are known for optimising desalting processes. For instance, demulsifiers are often added to minimise the rag layer and encourage the formation of separate hydrocarbon and aqueous phases. The application of an electrostatic field to the desalting unit may also be used to encourage the formation of separate phases.
However, the optimal conditions for operating a desalting process, and indeed for optimising other refinery processes, can vary greatly depending on the composition of the crude oil. It is therefore desirable to sample a crude oil before it is passed to a refinery process, such as a desalting process. This enables the refinery process to be optimised based on the particular crude oil that is being used.
Whilst crude oil may be sampled and tested before it is passed to the refinery, sometimes even before purchase, it is preferable to carry out on-line analysis of the crude oil. This analysis is often done using a sample loop in which a sample of crude oil is withdrawn from a process stream, analysed, and return to the process stream.
For example, GB 2170909 discloses a method for the on-line monitoring of crude oil in which a sample is isolated and circulated around a sample loop which includes a heat exchanger and a densitometer.
U.S. Pat. No. 8,497,683 discloses a spectroscopic sample analysis apparatus which includes an actively controlled heat exchanger in serial fluid communication with a spectroscopic analyzer, and a controller communicably coupled to the heat exchanger. The apparatus may be used to measure properties of the samples by spectroscopic methods, such as nuclear magnetic resonance, infrared, near infrared and Raman spectroscopy.
However, existing methods for the on-line sampling of crude oil suffer from a number of drawbacks. In particular, fouling of the sample loop can occur over time, e.g. around the analysis device, thereby reducing the accuracy of the crude oil analysis. This fouling has previously been thought to be caused by inorganic compounds. To protect the analysis device from inorganic solids, the crude oil is often passed through a filter before it is passed to the analysis device. However, in severe cases of fouling, filters in the sample loop may become plugged, preventing any analysis at all. As a consequence, optimisation of the refinery process to which the crude oil is passed may be compromised.
Accordingly, there is a need for an improved method for the on-line analysis of a refinery feedstock, such as a crude oil. There is also a need for an improved method for controlling a refinery process, such as a desalting process.